Making ventilation holes in a hockey mask style model
If your mesh is clean and subdivided enough you can make as many holes as you need with the following method. If it has not enough edge loops you can add some with W > Subdivide Smooth or add and apply a Subdivision Surface modifier.


Select four faces, press i to inset, and scale down the inset.
Press W > LoopTools > Circle
Delete the inner faces of your circle
Now you have a hole
Add a Solidify modifier to give thickness to your shape
Add an edge loop around your circle if you want a sharper edge


If you want a low-poly mesh you can create a low-poly version of your mask, bake your high-poly version, and, in the low-poly material, recreate fake holes with Normal or Bump maps.